Effects of metal substitution on the electric and thermoelectric properties in (Ni1−xMx)Mn2O4 (M = Zn and Mg)

The temperature dependence of the electrical resistivity and the thermoelectric power of (Ni1−xMx)Mn2O4 (M = Zn and Mg, x = 0, 0.1 and 0.2) are measured. The electric conductivity is described by a small polaron hopping mechanism. The activation energy for electronic conduction is increased above certain temperature. In the low temperature region the absolute value of thermoelectric power is increased by Mg substitution. The thermoelectric powers of all samples are found to change sign from negative to positive with increasing in temperature. This change of sign in the thermoelectric power may take care of the competition between spin-entropy term and hole conduction term caused by the charge transfer from O 2p to transition metal 3d orbitals.